View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Aberystwyth Research Portal Aberystwyth University Hydrocarbon source potential of the Tanezzuft Formation, Murzuq Basin, south- west Libya Diasty, W. Sh. El; El Beialy, S. Y. ; Anwari, T. A.; Batten, David Published in: Journal of African Earth Sciences DOI: 10.1016/j.jafrearsci.2017.03.005 Publication date: 2017 Citation for published version (APA): Diasty, W. S. E., El Beialy, S. Y., Anwari, T. A., & Batten, D. (2017). Hydrocarbon source potential of the Tanezzuft Formation, Murzuq Basin, south-west Libya: An organic geochemical approach. Journal of African Earth Sciences, 130, 102-109. https://doi.org/10.1016/j.jafrearsci.2017.03.005 General rights Copyright and moral rights for the publications made accessible in the Aberystwyth Research Portal (the Institutional Repository) are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the Aberystwyth Research Portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the Aberystwyth Research Portal Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. tel: +44 1970 62 2400 email: [email protected] Download date: 03. Oct. 2019 Accepted Manuscript Hydrocarbon source potential of the Tanezzuft Formation, Murzuq Basin, south-west Libya: An organic geochemical approach W.Sh. El Diasty, S.Y. El Beialy, T.A. Anwari, D.J. Batten PII: S1464343X17301097 DOI: 10.1016/j.jafrearsci.2017.03.005 Reference: AES 2841 To appear in: Journal of African Earth Sciences Please cite this article as: El Diasty, W.S., El Beialy, S.Y., Anwari, T.A., Batten, D.J., Hydrocarbon source potential of the Tanezzuft Formation, Murzuq Basin, south-west Libya: An organic geochemical approach, Journal of African Earth Sciences (2017), doi: 10.1016/j.jafrearsci.2017.03.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT Highlights 1. A wide variation of source richness and quality between Hot Shale and the Tanezzuft Formation is recognized. 2. Immature to early mature kerogen type II/III kerogen for the Hot Shale samples is reported. 3. The Tanezzuft samples are dominated by type III/II kerogen. 4. The Hot Shale and Tanezzuft Formation are of mixed organic matter input and different concentrations. MANUSCRIPT ACCEPTED ACCEPTED MANUSCRIPT 1 Hydrocarbon source potential of the Tanezzuft Formation, Murzuq 2 Basin, south-west Libya: An organic geochemical approach 3 1* 1 1 2 4 W.Sh. El Diasty , S.Y. El Beialy , T.A. Anwari , D.J. Batten 5 1 Mansoura University, Faculty of Science, Geology Department, Mansoura 35516, Egypt 6 2 Department of Geography and Earth Sciences, Aberystwyth University, Penglais, 7 Aberystwyth SY23 3DB, Wales, UK, and School of Earth, Atmospheric and Environmental 8 Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK 9 10 * Corresponding author: Dr. Waleed El Diasty, [email protected]; (+2) 0106 244 3492 11 12 Abstract 13 A detailed organic geochemical study of 20 core and cuttings samples collected from the 14 Silurian Tanezzuft Formation, Murzuq Basin, in the south-western part of Libya has 15 demonstrated the advantages of pyrolysis geochemical methods for evaluating the source- 16 rock potential of this geological unit. Rock-Eval pyrolysis results indicate a wide variation 17 in source richness and quality. The basal Hot Shale samples proved to contain abundant 18 immature to early mature kerogen type II/III (oil–gMANUSCRIPTas prone) that had been deposited in a 19 marine environment under terrigenous influence, implying good to excellent source rocks. 20 Strata above the Hot Shale yielded a mixture of terrigenous and marine type III/II kerogen 21 (gas–oil prone) at the same maturity level as the Hot Shale, indicating the presence of only 22 poor to fair source rocks. 23 24 Keywords : Rock-Eval pyrolysis, Hot Shale, Tanezzuft Formation, Silurian, Murzuq Basin, 25 Libya 26 27 1. Introduction 28 29 The MurzuqACCEPTED Basin is one of the largest Palaeozoic intracratonic basins in the North 30 African Sahara. It covers an area of about 350,000 square km in south-west Libya 31 (Davidson et al., 2000), extending southwards into Niger. The focus of this study is the 32 Silurian Tanezzuft Formation in three major oilfields, A, B and H within the NC115 1 ACCEPTED MANUSCRIPT 1 Concession in the basin (Fig. 1). The lower part of the formation is referred to as the Hot 2 Shale, a radioactive deposit that has a high gamma ray response in wireline logs, probably 3 as a result of a high content of uranium (Fello et al., 2006). This organic-rich shale is 4 thought to be the main source rock in the Murzuq Basin (Echikh and Sola, 2000; Sikander 5 et al., 2000; Craig et al., 2008), and is estimated to be the origin of 80–90% of all 6 Palaeozoic-sourced hydrocarbons in North Africa. These hydrocarbons have migrated into 7 various reservoirs, ranging from Cambrian to Triassic sandstones that are sealed by shale 8 intercalations. In the Murzuq Basin, Ordovician sandstones are the main reservoir rocks 9 (Lüning et al., 2000). 10 The NC115 Concession is located in the north-western part of the basin, about 11 1,330 km south-west of Tripoli. It is about 25,850 square km in areal extent, covering 12 approximately three-quarters of the area between 11° 30' and 12° 30'E, and 26° 8' and 26° 13 38'N. The B-NC115 Field is the most south-westerly of the fields examined. It is 14 approximately 50 km to the south-west of the A-NC115 Field. This in turn is approximately 15 10 km west of the H-NC115 Field (Fig. 1). 16 The application of organic geochemical analysesMANUSCRIPT of samples from the Hot Shale and 17 overlying deposits of the Tanezzuft Formation in the NC115 Concession is reported in this 18 paper in order to characterize the disseminated organic matter and to determine the 19 hydrocarbon generation potential of, and depositional environments reflected by, the 20 succession. 21 22 2. Geological background 23 24 The Murzuq Basin abuts the borders of Algeria, Niger and Chad. It is bounded to 25 the north by the Al Gargaf Uplift, to the east by the Tibesti High, and to the west by the 26 Tihemboka High (Echikh and Sola 2000). It lies to the south-east of the Ghadamis Basin, 27 from which it ACCEPTEDis separated by the Atshan Saddle, an anticlinal structure trending 28 approximately ENE–WSW, which leads into the western end of the Al Gargaf Uplift (Fig. 29 2). 30 The present structural framework of the basin reflects periods of uplift related to 31 Caledonian (Late Silurian–Early Devonian), Hercynian (Late Carboniferous–Permian) and 2 ACCEPTED MANUSCRIPT 1 Alpine (early Tertiary) events. The regional lineaments, NW–SE, are probably related to 2 late Precambrian Pan-African fault systems (Fig. 3), which largely controlled the early 3 Palaeozoic structural evolution of, and sediment accumulation in, the Murzuq Basin 4 (Klitzsch, 1995). 5 Deposition of the Silurian succession began after the melting of the Late Ordovician 6 ice sheets, which led to a major marine transgression that spread from the north, 7 culminating in a high-stand with deposition of the Llandovery–Wenlock Tanezzuft 8 Formation (Davidson et al., 2000). The formation was named by Desio (1936) after Wadi 9 Tanezzuft, between Ghat and Awaynat. It includes a thick shale unit that overlies the 10 Hirnantian (Late Ordovician) Mamuniyat Formation and underlies the Ludlow–Pridolí 11 (Late Silurian) Akakus Formation (Fig. 4). The lower part of the Tanezzuft Formation is 12 predominantly dark grey, fissile shale, and the upper part a monotonous argillaceous 13 sequence interbedded with thin sandstone layers. Altogether it is more than 700 m thick in 14 some wells in the NC115 Concession. 15 The basal part of the Tanezzuft Formation consists of the Hot Shale. It was referred 16 to as Rhuddanian black shale by Hallett (2002), theMANUSCRIPT Rhuddanian being the lowest stage of 17 the Llandovery Epoch (443.8–440.8 Ma; Gradstein et al., 2012). The high level of natural 18 radioactivity in some of the shales is the result of an increase in authigenic uranium, which 19 is readily recognized in well logs by high gamma-ray values (up to 400 API). High gamma- 20 ray intervals can also be related to lithologies other than hot shale, e.g., shale rich in detrital 21 K and Th (Lüning et al., 2000; Armstrong et al., 2005). The Hot Shale consists of black, 22 dark brown and dark grey, splintery, compact, micaceous, pyritic shale (Fig. 4). It is present 23 only in the B42 and H39i wells, where it is up to 45 m thick. The organic matter it contains 24 was probably a result of high algal productivity in a region of oceanic upwelling along the 25 North Gondwanan shelf margin (Finney and Berry, 1997).